In digital transmission systems, for the purpose of supplying the clock pulse there is generated in a control centre a master clock pulse which is transmitted via the digital transmission system to local digital transmission facilities. In each of the digital transmission facilities, a local slave clock pulse is generated which is synchronized to the received master clock pulse. The digital transmission facilities, e.g., data transmission facilities, are predominantly accommodated in subracks which have a central clock-pulse supply unit. The clock-pulse supply unit generates the slave clock pulse which is used as a central system clock pulse and, in a synchronous system, via the back panel which comprises a bus for system clock-pulse distribution, distributes it to all function units accommodated in the subracks.
If the functioning of the central clock-pulse supply is impaired or fails, this can affect the entire data transmission facility. For this reason, there is a change towards redundancy in the construction of the clock-pulse supply unit of the subracks. This means that a redundant clock pulse, which is non-dependent on the slave clock pulse, is generated locally through a second clock-pulse supply unit. If there is disturbance of the slave clock pulse it is possible to switch over to the redundant clock pulse. The redundant clock pulse is then the new slave clock pulse which assumes the function of supplying the clock pulse to the subracks. The switchover to the redundant clock pulse can be effected autonomously by the clock-pulse supply units or administratively, from the control centre.
The switchover can result in an interruption of a connection existing at that instant, e.g. a telephone connection, which can mean the loss of data. It is necessary to resynchronize the redundant clock pulse to the master clock pulse. This requires time. The interrupted connection must be re-established.